Concentrated solar power (CSP) is a form of alternative energy that is produced by capturing thermal energy of sunlight and converting it into forms of usable power. CSP systems typically include concentrators, such as parabolic dishes, mirrors, focal lenses, and/or other devices, that concentrate sunlight onto a receiver that absorbs thermal energy from the sunlight. Thermal energy absorbed by the receiver may then be converted into a desired form of power using a corresponding power conversion process. To generate electrical power, for instance, some CSP systems circulate a heat transfer fluid (HTF) through the receiver to carry solar thermal energy to a heat engine for producing mechanical work to drive an electric power generator. Other applications of CSP systems include propelling rockets operated by NASA and/or other space agencies as a replacement for chemical propulsion (e.g., burning fuels) and providing propulsion and on-board power for mini and micro satellites in space. It is also possible to heat an object with concentrated light and the object emission can be controlled (thermophotovoltaics).
The overall efficiency of CSP systems can be improved by raising the operating temperature and increasing the photothermal conversion efficiency of the receiver. The photothermal conversion efficiency of the receiver can be improved by increasing its solar absorptance (α) in the solar spectral region (e.g., for wavelengths (λ)≤2 μm) and lowering its thermal emittance (ε) in the infrared (IR) spectral region (e.g., for λ≥2 μm) at high operating temperatures (e.g., >650° C.). To achieve this type of performance, the surface of the receiver must be spectral selective or include a selective coating. However, spectral selective materials and coatings that perform well in both the solar and IR spectral regions at high operating temperatures can be difficult to identity and expensive to produce.
Some materials, including metals like gold and silver, for example, have low ε, hut also have low α and are therefore not optimum for use as spectral selective receiver coatings. Other materials, including transparent materials like oxides, nitrides, and carbides have high ε and low α and are also not optimum. Semiconductors, such as silicon (Si) and germanium (Ge), have been implemented in combination with broadband antireflective coatings to counteract their relatively high solar reflectance and increase α. However, these semiconductors have low IR reflectance resulting in high ε, and their performance degrades at high operating temperatures due to thermal oxidation.
Since materials having intrinsic optical properties that provide high spectral selectivity (e.g., α>95% and ε<10%) at temperatures higher than 500° C. are not found in nature, some manufacturers have implemented coatings comprising multiple materials in an attempt to improve the spectral selectivity of receivers. For example, coatings formed of multiple layers of metals (e.g., Mo, Ag, Cu, Ni, etc.) and dielectric materials (e.g., Al2O3, SiO2, CeO2, ZnS, etc.) stacked on the receiver surface have been implemented. However, inter-diffusion between the layers at high operating temperatures causes the performance of the receiver to degrade. Additionally, these coatings are manufactured in a vacuum environment and require precise control of layer thickness, which increases the cost of production.
Other material combinations that have been implemented in an attempt to improve spectral selectivity include ceramic-metal composites (cermets). Cermets comprise a mixture of metallic particles in a dielectric host and are deposited in layers on a metallic film. Cermet layers act as absorbers in the solar spectral region to increase α and as reflectors in the IR spectral region to reduce ε. Cermet layers serve as a graded index material that causes reduced reflection in the solar spectrum and increased absorptance in the IR spectrum as a result of electromagnetic wave interaction with metal particles and interference phenomenon. However, the performance of cermet layers degrades at high operating temperatures due to thermal oxidation that occurs when they are exposed to air. Cermet coatings are also costly to produce since they are made using vacuum fabrication techniques.
To reduce the effects of thermal oxidation at higher operating temperatures, some manufacturers have implemented vacuum enclosures that encapsulate the absorbing surface of receivers. However, the thermal stability of known receivers degrades beyond 350°-580° C. despite the implementation of vacuum enclosures. Additionally, the implementation of vacuum enclosures increases the design complexity as well as the cost to produce solar absorbers.
Some manufacturers have implemented surface texturing of materials as a way to increase solar absorptance of solar cells. In CSP applications, cermets and metals have been textured to achieve solar selectivity, which permits optical trapping of solar light through multiple reflections, resulting in higher solar absorptance. However, the performance of known textured surfaces drops due to oxidation at high operating temperatures.
Previously, sub-wavelength periodical tungsten (W) structures have been fabricated by fast ion beam etching. The fabricated sub-micron holes on tungsten can cause standing wave resonances that have been attributed to increase broad wavelength absorptance. However, this technique requires an expensive tungsten substrate and utilizes complex fabrication processes. Sub-wavelength structures on metal surfaces can increase solar absorptance due to surface plasmon absorption and also due to the surface behaving like graded index medium, thereby providing antireflection. IR reflectance from sub-wavelength structures on metal surface can be kept high when the IR wavelengths are longer than the dimensions (e.g., height and/or spacing) of surface roughness, causing the surface to appear smooth and radiate as a flat surface.
The disclosed method addresses one or more of the problems discussed above and/or other problems of the prior art.